1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device based on a nitride semiconductor.
2. Background Art
A nitride semiconductor device based on a gallium nitride (GaN) semiconductor has a larger bandgap than devices based on silicon (Si), and hence it is easier to realize a compact device with high critical electric field and high breakdown voltage. Its application to a power control semiconductor device can lead to a device with low ON resistance and low loss. Among nitride semiconductor devices, a heterostructure field-effect transistor (HFET) having an AlGaN/GaN heterostructure is promising for good characteristics with a simple device structure, where a GaN layer is provided as a channel layer and an aluminum gallium nitride (AlGaN) layer is provided as a barrier layer (electron supply layer).
In conventional GaN-based HFETs, the heterojunction interface between the GaN layer and the AlGaN layer is the (0001) crystal plane. Thus a two-dimensional electron gas (2DEG) at approximately 1×1013 cm−2 is generated at the AlGaN/GaN heterointerface by piezoelectric polarization. This polarization-induced 2DEG accounts for a large share of the 2DEG existing at the heterointerface. In such a semiconductor device where a 2DEG is generated by piezoelectric polarization, advantageously, carriers are generated without implantation of impurities. However, unfortunately, because piezoelectric polarization determines the sheet concentration of the 2DEG, the sheet concentration of the 2DEG varies with the surface potential of the AlGaN layer depending on the passivation material and process, and consequently the ON resistance varies.
Thus it is contemplated to prevent a polarization-induced 2DEG from being generated at the heterointerface. Prevention of polarization-induced 2DEG generation can be realized by setting the surface orientation of the crystal plane forming the AlGaN/GaN heterostructure to (1-101) or (11-20) (see, e.g., Masayuki Kuroda et al., “Normally-off Operation of Non-polar AlGaN/GaN Heterojunction FETs Grown on R-plane Sapphire”, Extended Abstracts of the 2005 International Conference on Solid State Devices and Materials, Kobe, 2005, pp. 470-471; and JP-A 2003-347315 (Kokai)). In this case, the GaN layer serving as a channel layer or the AlGaN layer serving as a barrier layer needs to be doped with n-type impurities for generating a 2DEG to serve as carriers. For achieving 2DEG sheet concentration comparable to that induced by polarization, the sheet concentration of n-type impurities needs to be approximately 1×1013 cm−2. However, if the sheet impurity concentration in the channel layer or the barrier layer is increased like this, the electric field distribution is varied by impurity ions, and the electric field concentrates on the edge of the gate electrode, unfortunately failing to achieve high breakdown voltage.